The foreign ministers of Ukraine, Russia, France and Germany have agreed that talks aimed at resuming a cease-fire between Ukrainian government and pro-Russian separatist forces should begin no later than Saturday.

The four top diplomats said in a statement after meeting in Berlin on Wednesday that the goal of the talks among representatives of Ukraine, Russia and the separatists would be "reaching an unconditional and mutually agreed sustainable cease-fire" to be monitored by the Organization for Security and Cooperation in Europe.

No venue or other details of the talks had been announced by early Thursday.

Ukrainian President Petro Poroshenko declared a unilateral cease-fire on June 20, extended it once, but ended it on Monday after nearly 30 soldiers were reportedly killed and dozens wounded during the truce.

Russian Foreign Minister Sergei Lavrov, speaking after the Berlin meeting, condemned the Ukraine leader's decision to end the unilateral cease-fire. He said that decision cost lives in eastern Ukraine and the "serious destruction of civilian infrastructure."

But he also vowed that Moscow would use its influence with separatist leaders to bring about a bilateral truce.

While some separatist leaders said they would observe a truce to run parallel with Kyiv's earlier cease-fire, the Ukrainian government accused the rebels of violating it more than 100 times. Poroshenko refused to extend the cease-fire a second time and resumed military operations in eastern Ukraine on Tuesday.

A spokesman for Ukraine's National Security and Defense Council, Andriy Lysenko, said Wednesday that at least five Ukrainian servicemen had been killed and 21 wounded over the previous 24 hours, while "hundreds" of rebel fighters died in the intensified fighting.

Lysenko said government forces had retaken three settlements from rebel forces and hit 120 separatist targets, while the separatists carried out 19 attacks on military convoys, checkpoints and positions.

Lysenko also claimed that Russia was blocking separatist fighters from fleeing into Russia.

Ukrainian military spokesman Oleksiy Dymtrashkivsky was quoted as saying that separatists in the Donetsk region hit a Ukrainian Su-24 fighter jet with a shoulder-launched missile, but that the aircraft managed to land with only one working engine.

Russia's Itar-Tass state news agency quoted a separatist source as saying government forces massively shelled the city of Kramatorsk starting Tuesday evening and into Wednesday.

Meanwhile, German Chancellor Angela Merkel said in Berlin Wednesday that she could not rule out further sanctions against Russia. The United States and its European allies accuse Russia of arming the separatists in eastern Ukraine.

Merkel said it was "regrettable" that Kyiv's 10-day cease-fire had not ended the violence.

On Tuesday, Russian President Vladimir Putin directly blamed Poroshenko for the renewed fighting. He also said Russia would continue to "vigorously" defend the rights of Russians abroad, using "all available means" including “international humanitarian law" and "the right of self-defense.”